Learn Hardcore French: Dans l’immeuble, l’escalier est sombre quand l’ampoule est cassée. - In the building, the stairway is dark when the light bulb is broken.

Starting the sentence this way sets the scene first. French often puts a place expression at the beginning to establish context, much like English can do with In the building, ...

So the structure is:

Dans l’immeuble = location
l’escalier est sombre = main statement
quand l’ampoule est cassée = time/condition-like clause

It is not unusual at all to begin a French sentence with the place where something happens.

Why is there a comma after Dans l’immeuble?

The comma separates the introductory location phrase from the main clause.

So:

Dans l’immeuble, = setting the scene
l’escalier est sombre... = main idea

In French, this comma is common when a short phrase like a place or time expression comes first, especially when the writer wants a small pause. It is similar to English:

In the building, the staircase is dark...

You may also sometimes see short opening phrases without a comma in everyday writing, but here the comma is completely natural.

Why is it l’immeuble and not le immeuble?